1. Field of the Invention
The present invention relates to an optical switch, and more particularly to a thermo-optic switch using a small drive power while exhibiting a reduction in the coupling loss caused by the coupling to optical fibers and a switch speed of several hundred microseconds or less. The present invention also relates to a method for manufacturing the thermo-optic switch and a method for changing an optical line switching using the thermo-optic switch.
2. Description of the Related Art
Examples of optical switches, thermo-optic switches, or electro-optic switches, incorporated by reference herein, are found in U.S. Pat. No. 5,121,450 to Elliot Eichen et al. entitled Fiber Optical Y-Junction, U.S. Pat. No. 5,418,868 to Leonard G. Cohen et al. entitled Thermally Activated Optical Switch; U.S. Pat. No. 5,623,566 to Hyung J. Lee et al. entitled Network With Thermally Induced Waveguide, U.S. Pat. No. 5,970,186 to John T. Kenney et al. entitled Hybrid Digital Electro-Optic Switch, and U.S. Pat. No. 6,067,387 to Min Cheol Oh et al. entitled Electro-Optic Polymer Waveguide Device For Decreasing Driving Voltage And An Optical Loss And Method Of Making The Same. 
Generally, a thermo-optic switch is a device for changing an optical line using a variation in the refractive index of the material of the device depending on a variation in temperature applied to it, the material of the device. Thermo-optic switches are mainly classified into a Mach-Zehnder interference type, a directional coupler type, and a digital type.
FIG. 1 illustrates an example of a digital thermo-optic switch. As shown in FIG. 1, the digital thermo-optic switch includes a substrate 10, a lower clad layer 120, a core layer 130, an upper clad layer 140, and a heater 150.
FIG. 2 is a schematic view illustrating the operation principle of a digital thermo-optic switch using a mode evolution principle. The digital thermo-optic switch has a branched waveguide structure having branched waveguides 210. Electrodes 220, which are made of a metal, such as gold, exhibiting a superior thermal conductivity, are formed on each branched waveguide 210. When heat is applied to one of the electrodes 220, it is transferred from the electrode 220 to the branched waveguide 210 arranged beneath the electrode 220, so that the branched waveguide 210 exhibits a reduced effective refractive index. As a result, a difference of effective refractive index occurs between the branched waveguides 210. Accordingly, an input light is switched to the branched waveguide 210 in accordance with a mode evolution thereof. In Mach-Zehnder interference or directional coupler type thermo-optic switches using an inter-mode interference phenomenon, a light switching operation is achieved by virtue of a line length difference between two branched waveguides resulting from a difference between the effective refractive indices of those branched waveguides.
Thermo-optic switches may be implemented using waveguides having an embedded structure or a rib structure. A thermo-optic switch, which has the embedded structure, is manufactured using materials exhibiting a refractive index difference ranged from 0.3% to 0.6% in order to reduce the coupling loss caused by the coupling to optical fibers. Typically, the thermo-optic switch has a core thickness of 6 to 8 xcexcm and a total waveguide thickness of 25 to 40 xcexcm. In this case, an optical fiber coupling loss of 0.5 dB/facet or less is exhibited.
FIG. 3 illustrates a cross section of the thermo-optic switch having the embedded structure. As shown in FIG. 3, the thermo-optic switch includes a heat sink 310, a clad 320, branched waveguide cores 330, and electrodes 340. In such a thermo-optic switch having the above mentioned embedded structure, heat applied to one of the electrodes 340 is transferred to an associated one of branched waveguide cores 330 in a thickness direction in an isotropic fashion. For this reason, where the thermo-optic switch has a large total waveguide thickness, heat is not only transferred to a desired one of the waveguides, but also transferred to the remaining waveguide in a considerable amount. As a result, it is difficult to obtain an efficient thermo-optic effect. Furthermore, the transfer of heat to the heat sink 310 arranged beneath the waveguides is carried out at a lowered rate. For this reason, the time taken for the applied heat to be completely discharged out of the waveguides is also unacceptably lengthened. In other words, the switching speed of this type of thermo-optic switch is too slow.
FIG. 4 is a cross-sectional view illustrating a thermo-optic switch having the rib structure. As shown in FIG. 4, the thermo-optic switch includes a heat sink 410, a lower clad 420, a core 430, an upper clad 440, and electrodes 450. In the case of thermo-optic switches, which have the rib structure, materials exhibiting a refractive index difference ranged from 1% to 10% are typically it used. Where materials exhibiting a high refractive index difference are used to manufacture a thermo-optic switch having the rib structure, it is possible to obtain a total waveguide thickness of 15 xcexcm or less because the clad of the thermo-optic switch affected by an evanescent field formed in the thickness direction of the waveguides can be formed to be very thin. In this case, accordingly, heat applied to one of the electrodes 450 is transferred only to a desired one of the waveguides of core 430. As a result, it is possible to greatly reduce the transfer of heat to the remaining waveguide. Since the total waveguide thickness corresponds to xc2xd the total waveguide thickness in the general embedded structure, the distance between each electrode and the heat sink is correspondingly short. As a result, an easy heat discharge is obtained. In addition, the drive power used for the thermo-optic switch can be considerably reduced. There is a disadvantage, however, in that a large coupling loss occurs in the thermo-optic switch having the rib structure due to a mode size difference from optical fibers to which the thermo-optic switch is coupled. For this reason, it is difficult to manufacture a thermo-optic switch having a small coupling loss.
As is apparent from the above description, thermo-optic switches, which have an embedded structure or a rib structure, have the following problems. That is, in the case of a thermo-optic switch having the embedded structure, which has an advantage in that the coupling loss caused by the coupling to optical fibers can be reduced to 0.5 dB/facet or less, it is difficult to achieve an efficient switching operation because the distance between each electrode and each associated waveguide is considerably large because of a large total waveguide thickness of 25 to 40 xcexcm. As a result, the thermo-optic switch exhibits a relatively low switching speed. In the case of a thermo-optic switch having the rib structure, it can have a small total thickness of 10 xcexcm or less by virtue of a high refractive index difference exhibited in the rib structure. Accordingly, the drive power used in the thermo-optic switch can be reduced, as compared to that used in the thermo-optic switch having the embedded structure. Also, there is an improvement in switching speed. However, the thermo-optic switch having the rib structure has a disadvantage in that a large coupling loss occurs due to a mode size difference from optical fibers to which the thermo-optic switch is coupled. For this reason, it is difficult to manufacture a proficient thermo-optic switch having a small coupling loss.
Therefore, an object of the invention is to provide an improved thermo-optic switch.
Another object of the invention is to provide a thermo-optic switch which has a rib structure exhibiting a coupling loss, caused by the coupling to optical fibers, reduced to 0.5 dB/facet or less and having a reduced distance between each electrode thereof and a heat sink thereof, so that it is capable of using a small drive power while exhibiting a switching speed of several hundred microseconds, and to provide a method for manufacturing the thermo-optic switch.
Yet another object of the invention is to provide a method for manufacturing a thermo-optic switch which has a rib structure exhibiting a reduced coupling loss.
A further object of the invention is to provide a method for changing an optical line using the thermo-optic switch.
In accordance with one aspect, the present invention provides a thermo-optic switch having input and output terminals respectively connected to optical fibers, comprising: a substrate having etched portions at regions respectively corresponding to the input and output terminals; a lower clad layer formed over the substrate, the lower clad layer having an input taper formed at the region corresponding to the input terminal and adapted to convert a circular mode, input from the optical fiber connected to the input terminal, into an oval mode having a rib shape, and an output taper formed at the region corresponding to the output terminal and adapted to convert the oval mode into a circular mode allowed to be input to the optical fiber connected to the output terminal; a core layer formed over the lower clad layer and provided with branched waveguides having a rib structure, the branched waveguides selectively receiving the oval mode from the input taper and outputting the received oval mode to the output taper; an upper clad layer formed over the core layer; and switching electrodes formed on the upper clad layer and selectively activated to apply heat to an associated one of the branched waveguides in such a fashion that an effective refractive index difference occurs between the branched waveguides, thereby causing the branched waveguides to selectively receive the oval mode from the input taper.
In accordance with another aspect, the present invention provides a method for manufacturing a thermo-optic switch having input and output terminals, comprising the steps of: (a) preparing a substrate, and etching portions of the substrate respectively corresponding to the input and output terminals; (b) forming a lower clad layer over the substrate; (e) forming an input taper and an output taper at portions of the lower clad layer respectively corresponding to the input and output terminals; (d) forming a core layer over the lower clad layer formed with the input and output tapers; (e) forming branched waveguides having a rib structure at the core layer in such a fashion that the branched waveguides are arranged between the input and output tapers; (f) forming an upper clad layer over the core layer formed with the branched waveguides; and (g) forming switching electrodes on the upper clad layer.
In accordance with another aspect, the present invention provides a thermo-optic switch having input and output terminals respectively connected to optical fibers, comprising: a substrate having an input taper formed at a region corresponding to the input terminal and adapted to mode conversion by converting a circular mode, input from the optical fiber connected to the input terminal, into a flattened mode, or oval mode having a rib shape, and an output taper formed at the region corresponding to the output terminal and adapted to convert the oval mode into a circular mode allowed to be input to the optical fiber connected to the output terminal; a lower clad layer formed over the substrate; a core layer formed over the lower clad layer and provided with branched waveguides having a rib structure, the branched waveguides selectively receiving the oval mode from the input taper and outputting the received oval mode to the output taper; an upper clad layer formed over the core layer; and switching electrodes formed on the upper clad layer and selectively activated to apply heat to an associated one of the branched waveguides in such a fashion that an effective refractive index difference occurs between the branched waveguides, thereby causing the branched waveguides to selectively receive the oval mode from the input taper.
In accordance with another aspect, the present invention provides a method for manufacturing a thermo-optic switch having input and output terminals, comprising the steps of: (a) preparing a substrate, and forming an input taper and an output taper at portions of the substrate respectively corresponding to the input and output terminals; (b) forming a lower clad layer over the substrate; (c) forming a core layer over the lower clad layer; (d) forming branched waveguides having a rib structure at the core layer in such a fashion that the branched waveguides are arranged between the input and output tapers; (e) forming an upper clad layer over the core layer formed with the branched waveguides; and (f) forming switching electrodes on the upper clad layer.
In accordance with another aspect, the present invention provides a method for changing an optical line using a thermo-optic switch having input and output tapers respectively coupled to optical fibers, branched waveguides having a rib structure arranged between the input and output tapers, and electrodes adapted to allow the branched waveguides to be selectively switched to the input and output tapers, comprising the steps of: (a) converting a circular mode, input from the input taper-end optical fiber to the input taper, into an oval mode having a rib shape; (b) activating a selected one of the electrodes, thereby generating heat from the selected electrode, and transferring the heat to a selected one of the branched waveguides in the vicinity of a branching point of the waveguides, at which the waveguides are coupled to the input taper, thereby switching the optical line for the oval mode to the branched waveguide, to which no heat is applied, by virtue of a mode evolution; and (c) changing the oval mode having the rib shape into a circular mode, corresponding to the output taper-end optical fiber, during a passage of the oval mode through the output taper.